Apparatus for amplifying electric oscillations



June 3, 1930., L. M. HULL APPARATUS FOR AMPLIFYING ELECTRIC OSCILLATIONS Original Filed Jan. 9, 1925 2 sheets-shee 1 M INVENTO BY Q I W ATTORNEYS.

June 1930-- L. M. HULL 1,760,872

APPARATUS FOR AMPLI'FYING ELECTRIC OSCILLATIONS Original Filed Jan 9, 1925 2 SheetsSheet 2 6 9, M INVENTOR,

I BY

ATITORNEYS.

Patented June 3, 193% PATE OFFica I LEWIS M. HULL, F MOUn'rAm LAKEs, NEW JERSEY, ASSIGNOB. TO RADIO FREQUENCY LABORATORIES, INoORroRA'rED, or noon'ron, NEw JERSEY, A CORPORATION or I g i v q I Original application filed January9, 1925, Serial No. 1,471. Divided and this application filed March 24,-

NEW" JERSEY APPARATUS FOR mrrrrvrne ELECTRIC oscrLLA'rrONs v 1930. Serial No. 438,591:

This invention relates ad apparatus for is'not equal to the product of the, amplification factors ofv'the several stages. This failure to secure the theoretical voltage amplification from a plurality of stages is particularly pronouned at the higher frequencies from 500 object of the invention is to provide a plu- 'rality of. tuned amplifying stages which are so constructed and arranged that the input reactance of each amplifying stage is indeto 1500 kilocycles per second but is marked pendent of the value of the variable reactance in'the input 'circuit of the succeeding at the intermediate frequencies lying between 30 and 100 kilocycles' If the amplifyin'g tubes in a number, n, of like cascaded stages were employed with full efficiency, the overall voltage amplification of the system would be numerically equal to the voltage amplification per stage raised to the nth power. This I define as true cascade amplification.

The amplification obtainable with prior systems and radio receivers has fallen far short of such a figure. With a radio-frequency transformer-coupled amplifier, as

I sem'bled in the usual fashion and with the I stages in cascade.

cade System of the intermediate frequencyv tendency towards self-oscillation in individual stages controlled by the-introduction of variable losses in the input stage, I have found, experimentally, a voltage amplificas tion of 100 for asingle stage, 200 for twosimilar stages in cascade and 310 for three similar In another typical castype,operating at afrequency of 30 kiloc'ycles, the voltage'amplification for one stage is of the order of 30, for two stages 450, and for three stages 900. If the amplification for one stage is termed A, the amplification observed at the higher frequencies corresponds respectively, to A ,,,A and A for one, two and three stages; and the amplification at inter mediate frequencies was A A and A, re-

I spectively, for one, two and three stages. The

significance of these figures is not in the absolute values of the amplification, which vary with the types of tubes and'coupling thereof which are hereinafter descri ed and I claimed.

Contrary to the universal opinion, based upon evidence similar to that presented above in quantitative form, that the voltage amplification of each added radio frequency stage is progressively smaller and that no real'adv'antage is gained by using more than two, or at most three, stages of amplification,

I have demonstrated that a number of stages may be so cascaded as'to provide an overall voltage amplification which increases expone'ntially with the number of stages.

To secure this cascade amplification, I have found two definite conditions which must be fulfilled: first, each stagemust be a true Oneway' repeater; second, the interaction between non-adjacent stages due to stray electrio and magnetic couplings and to small voltage drops in common battery leads, must be reduced to insignificance. In other words, if each stage is so arranged and adjusted that the input circuit of any given audion repeater has no appreciable effect upon any circuit ahead of this input circuit in the amplifier,

and if the current in this input circuit afiects r all circuits beyond it in the cascade ampliof a true one-way action in each of a series of stages is beneficial in a different way: it renders the effective input reactance of any given stage independent of'the constants of each of the succeeding stages, including the output circuit of the particular stage in question, with the result that when the, coupling units of each include a tuned circuit, the tuned circuits of all the stages can be made mutually resonant by means of rea'ctances external to the several audions which are all equal.

In previous attempts to tunes-plurality of similar stages by a single control the'progressive diiferences whichmay exist in the input reactances of a series of cascaded stages, which differences are prese nt even though the external tuning reactances are equal for all the stages, has apparently been overlooked, with' the result that the bend'of frequencies through which the different stages are even approxim'ately in resonance with each other is extremely limited.

- I also gain increased selectivit in a series of cascaded stages owing to the act that the variation of effective input capacity with frequency which is known to exist when the output circuitof an amplifying stage is allowed to react upon the input circuit of that stage,

. is absent in an amplifier made up of one-way structure.

stages. W I

For a better understanding of my invention, reference is had "to the accompanying drawings, in which, f V, Y

* 1 is a diagramshowi-ng the schematic organization of my invention.

Fig.2 is a circuit diagram of acne way'am plifying stage suitable for use with my invention. I

Fig. 3 is a broken perspective view of one physical embodiment of a plurality ,of cascaded one-way stages, and 7 Fig. 4 is a circuit diagram of a radio re- :ceiver including a pluralit, of cascaded one way radio" frequency amp ifying stages, the first or input amplifying stage being adapted. for use with an antenna or capacity collecting In the schematic diagram of Fig. 1 an am-' .plifying system according to my invention is shown embodied in a'radio receiver in which the antenna-ground circuit 1, 2, works into a cascadedseries of unidirectional amplifying stages 3, 4, 5 and 6, which work in turn into a detectorsta e,7.

True uni irectional action of the cascaded stages such as is contemplated in the present ally not be unidirectional for several reasons,

as follows:

1. On account of the electrostatic coupling .due to the capacity between grid and plate electrodes of the audion and their-connecti ons; p 2. On account of the flow of ions or electrons from the filament to the grid of the audion;

3. On account of the coupling due to the flow of ,the plate (thermionic) current through the filament;

4. On account of the charge induced upon the grid electrode by the varying space charge;

5. On account of any incidental coupling between the coils or condensers and their connections on either side of the repeater tube. v 90 For purposesof discussion, I shall separate all amplifying stages into two general classes, which I'define as follows:

First, one-way stages, in which retroactive and-interactive couplings due to all five of the above-named causes have been substantially eliminated; second, neutralized stages, in which the coupling of the first type only (electrostatic capacity between grid and plate.) has been eliminated. When two new 09 tralized stages are cascaded, if the losses in the interstage coupling transformers are sufficiently high, the combination may be nonoscillatory over a substantial range of freu .quencies. .But when two or more one-way 1 stages are cascaded, not only is the combination non-oscillatory, regardless of the nature of the couplings used, but true cascade amplification results. The failure of conventional amplifiers comprising two neutralized 10 stages so proportioned as to be non-oscillatory, to eliminate all important retroactive couplings accounts for the relatively high overall amplification in such systems. A1-

, though the residual regeneration present in two neutralized stages may produce an overall amplification for these two which is greater than thatwhich would be obtained if the same two stages were converted to the one-way type, the significant fact is that not even-an approximation of true cascade amplification is obtained when more stages are added to the neutralized amplifier.

A typical one-way amplifying stage is shown diagrammatically in Fig. 2. The in- Pat transformer L L is tuned by a condenser O in the usual manner. The audion T comprises a three electrode amplifying tube having capacity O between the grid and plate P. The output circuit of the stage includes the primary L of the transformer 1L L,, the secpoint, Y, of-the secondary of the input transformer is connected to a filament lead F- through a coil L which is coupled to L and a grid biasing battery C. By low-point of a circuit element or impedance I mean that point of the element which is at the lowest alternating current potential, or, in an audion circuit, most closely approaches the alternating current potential of the audion filament. The A and B batteries and the rheostat B may be of the usual formsi- N As has been described in the above-mentioned Ballantine application, this arrangement of input and output networks constitutes a bridge having the capacity arm. C and a second capacity arm 0 between the grid and filament terminals which second arm may comprise the capacity between the electrodes or their connecting wires, or an external condenser. The points G, P and F, are three corners of the bridge system and the fourth point is the point X in the output inductance L Due to. the coupling between L and L, the point Y in the input circuit is at the same A. C. voltage as some point X'of the output circuit, and the input of the stage is effectively across Gr and X, which are at opposite'corners of the bridge; By adjusting G to provide the proper capacity and thus balancing the bridge, the reaction of the output upon the input circuit through C is prevented since the output is across P and F, the

other corners of the bridge. Theoretically the bridge can be balanced independent 'of frequency with any .value of coupling between the coils L and L since a decrease in this coupling, with a given pair of coils, is entirely compensated in obtaining a balance by an increase in the value of caphcity C,,. I have found'in one specific practical embodiment of my invention using this type of circuit with an audion of the type known commercially as UV201A that a suitable value for the coefficient of coupling between coils L and L is K=0.25, where K is defined by the relationz K: k. q Y C The numerical ratio between inductances Lr, and L should be in this case L /L =4 to 4.5.

A bridge balance may be obtained with a capacity at C of the order of 40 micro-microfarads. If this balance is obtained in the absence of thermionic space. currents within the audion, then the electrostatic coupling through C only is eliminated andthe stage falls in the class of'neutralized stages. My

invention is not, of course,-limited to these or any other specific constants.

In OI'dBI'fl'iQ develop, his system further,

into a one-way stage, the other causes of retroactive currents are eliminated as follows:

The grid-filament current, which manifests itself as a resistance shunted across 0., iselim- I inated by a C battery or other. means for biasing the grid negatively; If a C battery 1s not used, the efiect of the grid-filament current may be compensated by the use of extra balancing resistances in'the-bridge arms as explained in the Ballantine application. The coupling arising from the flow of plate (thermionic) current in the filament is eliminated by the use of a high-capacity filament by-pass condenser C shunted across the filament terminals. Incidental couplings between the input and output circuits are eliminated by shielding around the coils, condensers, and

high-potential connecting wires.

The general technique of radio and audiofrequency shielding is-well known in the art,

as is the practice of shieldingcertain parts of amplifiers from each other. I have found,

however, certain special types'and arrangem'ents of shields to be peculiarly adapted for the provision of one-way action in cascade amplifier circuits. An individual shield for each transformer and its associated leads is absolutely essential for one-way action. It is also desirable to shield the tuning condensers from each other and from the'a'udions and associated parts. The preferred form of shlelding is indicated diagrammatically by dotted lines in Fig. 2, which lines define four enclosed spaces S S S and S within which are located, respectively, rthe input 4 transformer L L input tuning condenser C the output transformer L L which com prises the input transformer of the succeeding stage, and the tuning condenser G for tun- L5- 0 i v These shields may be formed of highly conductlng metal, of a thickness which depends upon the constants of the metal. 'lVith coupling transformers having strai ht' solenoiinches be tween centers, sheet copper or sheet brass.

dal windings, positioned about about 0.02 inches thick is suitable.

A practical physical embodiment of the principahpa'rts of a stage andof the shields eral parts are identified by the same reference characters as are used in the diagram of or cubicles within which the elementsare placed.

In Fig. 3, the input stage is shown at the right of the first complete amplifyin sta e it being understood that additional aid si iii:

ilarly constructed amplifying sta' es will ordinarily 'be arranged to the lie 'of these stages. The particular arrangement illus v is illustrated in Fig. 3, in which view the sev- 1 trated intended for use in a cabinet of tected a change in the effective grid late lindrical' transformers being arranged'with with a cold'filament give spurious results,

the usual type and with the aligned ends of capacity when the filament. was turne on, the boxes adjacent the usual vertical panel. which amounted to of the original elec- The fixed inductances are arranged at the trostatic capacity between electrodes.

lower part of the assembly, the-several cy- Thus the old methods of. securing a'balance their axes horizontal and normal to the ends and do not produce the desired one-way effect of the shields S S which will be adjacent even though the circuit purported to be balthe panel. Theshields S S which house anced includes all physical elements necesthe tuning condensers C C are shorter than sary for securing an accurate balance. An the boxes S S and are arranged abovethe accurate balance can be secured only when latter at their forward ends. The socket for. due consideration is given to the effects of audion T, the battery leads and the conthis space charge. I obtain a true one-way densers C C arepositioned on or above the vaction by adjusting the circuit constants to shelf which is provided by the rear portion provide abridge balance on each individual of the shield boxes S 8,, it being obvious stage with the filaments lighted and with that these various parts are insulated from the normal thermionic space current flowthe shields. An additional shield S is proing, suitable apparatus for and the method vided, this taking the form of a flat plate of securing a hot tube balance being dewhich is positioned aboveand parallel to the scribed by me in QST, August, 1924, pages upper surfaces of the shields S S4, and 32 to 34. which hasspaced openings above the several The physical arrangement of an amplifier tube sockets. having more than two cascaded stages will The second of the two requirements for be apparent from a consideration of Fig. 3,8 cascade amplification, vizz-isolation of the and 4 is a circuit diagram of an arm several stages, is accomplished with respect plifier, specifically a radio receiver, having to parasitic 'couplingsby the shields which three stages in addition to the first amplifysurround the coupling units, since these ing or input stage, and the audion detector. shields serve to isolatethe units individu The several tubes of the foun amplifying ally. With certain arrangements, I have stages are indicated by num erals 1 to 4;, refound it advisable also to enclose the audion spectively, and the detector 'audion by nutubes in individual shields S i In addition metal 5 The input or first amplifying stage to this, it is necessary to bring to ground differs from the succeeding stages as the repotential all low voltage battery leads by actance of the first input transformer will densers.

due to this mobile space charge which conproper position to balance the separate' stages.

the liberal use of high-capacityby-pass-conbe influenced by the properties of the col- 7 v I lecting structure with which the receiver. is It will be noted that the tuning condensers associated. In addition to the input trans- 2 and C are shielded from their respective formerjL L and tuning condenser'O "an inductances, L and L and that the wiring adjustable reactor, as the condenser C isprowithin a stage is positioned within the shields vided in series with inductance L the an which house the parts of the stage. The tube tenna and ground G being connected across socketand battery wires are located outside C L- The input and output networks asof the boxes but the leads from the plate sociated with the subsequent, "amplifyin and grid terminals of the socket pass directly audions are identical, and are constructed into the boxes S S and the exposed p'or- 'andarranged as described in detail in 0011-. tions of these leads are kept as shOrtas is nection with Figs. 2 and 3. In cascadinga possible. a g A plurality of stages, common batteries are I have enumerated all of the features necesused, and additional by-pass condensers of sary to provide one-way characteristics. for large capacity C and are provided across the amplifying stage of F igsf2 and?) except the B and C batteries, respectively. Separate that one which concerns the retroactive c6uby-pass condensers G are provided for each" pling due to the space charge'in the audion. stage and individual balancing condensers (3 As pointed out in .the copending Ballantine are of course necessary, the condensers C beapplication, Ser. No. 720,708, the electric flux ing o' nce adjusted and preferably clamped in stitdtes the-thermionic plate, current of the' The severaltuning condensers C are ar- .audion, manifests itself as a change in the ranged for operation by 'a. single control, effective grid-plate. capacity of the audion. which as indicated in Fig. 3, com rises a Whenthe filament of the audionis heated shaft arranged parallel to the fronto the set, and the steady plateivoltage turned on the suitable insulation being provided between effective capacity between grid and plate is .the sections of the shaft to which the several different from theelectrostatic capacity be 1 sets of movable plates are attached. Due to tween these/electrodes. I-wish to emphasize the fact that the effective input reactances that this is no minor, sec'ond-ordereffect of stages between terminals G and Y, note In an ordinary receiving audion, I have de- Fig. 2, are equal when the oouplingtrans one-way repeaters.

formers have primary and Secondary inductances, respectively, equal to each other,

the one-way stages including audions 2, 3 and 4: may be simultaneously tuned to resonance by similar variable condenser-s G which are so coupled as to move in unison;- Although the effective input reactance of the input audion 1 is affected bythe particular current collecting structure which may be' used, the

, effect of the arbitrary value of subh a structurecan beeliminated by a suitable adjustment of the compensating condenser C As the input stage is also a one-way repeater,

the adjustment of .61, is substantially independent of frequency and in actual practice no variation in the setting of C can be observed when the latter is adjusted, for a given antenna, for best reception of the highest or the lowest frequencies within the broadcast range. The four amplifying stages may be concomitantly tuned to resonance by a single control, even though the tuning of the individual stages be extremely sharp, an operation which has not been possible with pre- Vious constructions which include a plurality of sharply'tuned amplifying stages. In prior constructionsusing a single control, no account has been taken of the fact that even though the constants of all external react- 3 ances are equal, a progressive difierence may exist between the effective input reactance of each amplifying tube, with :the result that there has been only one or two settings of the single control at which all stages were on V 3 the resonance peak. At other settings or frequencies all but one of the stages have been slightly off the resonance peak. Thepresent invention avoids this" difliculty since the input stage and all subsequent stages will be 40 tuned to the resonance peak when the stages are one-way amplifiers and the external reaotances of the several stages are identical.

In its broader aspects, the present invention comprises the cascading of a-plurality of Contrary to previously accepted theories, I have ,found that the elimination of such incidental feed-back a's has increased the amplification in one or two stages, has not resulted in lessened amplification for a plurality of stages but has had the surprising result ofsecuring an actual amplification equal to the maximum which was tlfeo ,retically obtainablewith a given numberof similar stages. Ihave built cascaded amplifiers having four resonant one-way ,stages operating'at frequencies between 500 and 1500 kc. in which each stage was constructed.

and adjusted. in accordance with my inven- \tion as described herein. The amplification eofor' a single stage was 7, and the observed overall amplification of the fourstages was, within the limits of experimental error, equal toZor 2491.

It is apparent that the invention is not limited to the specific circuit details and arrangements since other means are now known or may be devised for eliminating'some of the causes of retroaction in a stageor between stages. .r

This application is a division of my copening application SenNo. 1,471, filed Janj9,

I claim:

1. A radio frequency amplifier comprising a plurality of cascaded vacuum tube stages of the'type including means suppress for said tuning condensers, for each of saidg transformers, andfor each vacuum tube. 3. An electrical amplifier stage comprising in combination a vacuum tube provided with plate, grid, and filament electrodes, an input and an output transformer, a circuit associated with saidrstage to neutralize undesirable coupling between the outputtand input circuits. thereof and capable of. adjustment.

under operating conditions, means for overcoming undesirable effects of electron flow ,from filament to grid, a b'y-pass condenser for said filament, and individual shields for each of said transformers and for said vacuum tube, the elements of said combination being so associated and adjusted that the amplifier stage is unidirectional inac'tiom 1 4. An audion amplifier system comprising a plurality of audion amplifier stages dis posed in series in cascaded arrangement; each of said amplifier stages being constructed and arranged for one-way forward action only, and comprising, in combination,"meaiis for reducing the efiect of retroactive, currents I due to capacity between the control element and anode of the audion tube, means for imparting a negative potential tothe control 4 element with relation tovthe cathode in orderto reduce the flow of electrons to the control element, means for reducing the effects of the charge induced upon the control element by tlie presence of space charge existing within the audion when the cathode is in emitting conditionyand means for shielding the stage to substantially prevent coupling external; thereto not desired for the forward action thereof; whereby said audion amplifier-system is caused to operate with approximately' true cascade amplification, substan tially in the manner-described.

' An audion amplifier system comprising a plurality of audion amplifier stages disposed in series in cascaded arrangement; each of said amplifier stages being constructed and arranged for one-Way forward action only, and comprising, in combination, means for reducing the effect of retroactive currents due to capacity between the control element I and anode of the audion tube, means for imparting a negative potential to the control element with relation to the cathode in order to reduce the flow of electrons to the control element, means rendering the direct current potential between said control element and cathode substantially independent of the flow of spaced current through said cathode, and means for shielding the stage to substantially prevent coupling external thereto not desired for the forward action thereof; whereby said audion amplifier system is caused to operate with approximately true cascade amplification, substantially in the manner described.

6. An audion amplifier system comprising a plurality of audion amplifier stages disposed in series in cascaded arrangement; each of said amplifier stages being constructed and arranged for substantially one-way forward action only, and including, in combination, an audion tube device and a plurality of impedances forming an alternating current Wheatstones bridge having two conjugate arms comprising respectively the input circuit of the sta e and the output circuit thereof, and four alancing arms characterized by the fact that the product of the impedances of one pair of non-adj acent balancing arms is substantially'equal'to the product of the impedances of the remaining pair of non-adjacent balancing arms throughout the frequency'range of the stage, means for imparting anegative potential to the control element with relation to the cathode in order to reduce the flow of electrons to the control element, means for making the cathode a substantially equipotential surface with regard to'alternating currents to be amplified, means for reducing the effects of the charge lnduced upon the control element by the presence of space charge-existing within the audlon when the cathode is in emitting condition, and means for shielding the stage to substantially prevent cou ling external thereto not desired for the orward action thereof; whereby said audion amplifier system is caused to o erate with approximately true cascade amp ification, the manner described.

In testimony whereof, I aifix my signature.

LEWIS M. HULL.

substantially in 

